Disrupted neural pathways contribute to the emergence of abnormal intralimb coordination following stroke. This can result in an abnormal regulation of limb mechanics and related functional impairments. Stroke results from a brain infarct, but there is evidence that altered spinal function also may contribute to post-stroke motor deficits. To date, most studies into limb mechanics and reflex function have focused on passive, single joint conditions, but these studies do not address the conditions encountered during functional tasks that require active coordination of multiple muscles and joints. Hence, this study will focus on active regulation of multijoint mechanics following stroke. Our goals are to investigate how regulation of multijoint arm mechanics is constrained following stroke in comparison to age-matched control subjects and to determine if there is evidence for reflex contributions to these constraints. Arm mechanics will be quantified using estimates of 3-dimensional (3D) endpoint stiffness, which describes the relationship between perturbations imposed at the hand and the forces generated in response. As such, endpoint stiffness characterizes the mechanical properties of the arm at the point of contact with the environment. A 3D robotic manipulator will be used to measure endpoint stiffness and to assess stretch reflexes. Twenty adult subjects with stroke and 20 age-matched controls will be tested. Our first hypothesis is that stiffness regulation during voluntary force generation is constrained following stroke. Testing this hypothesis will provide insight into the neural constraints limiting arm function after stroke. Our second hypothesis is that stretch-sensitive reflexes elicited by perturbations of multijoint posture are enhanced following stroke and that reflexively elicited patterns of muscle activation mirror abnormal coordination patterns previously reported for voluntary activation. Testing this hypothesis will provide insight into the mechanisms constraining arm use after stroke and may prove useful for designing appropriately targeted rehabilitation interventions. According to CDC's Healthy People 2010, stroke is a leading cause of physical disability in the United States. This study will examine possible mechanisms of impaired limb function in order to better target patient-specific rehabilitation interventions for stroke survivors.